This invention relates to a metering device for metering and feeding liquids to be mixed, in particular for metering and feeding chemical components which are capable of reacting according to precise stoichiometric ratios, for example, to form polyurethane mixtures to be fed into a mold.
It is well known to feed stochiometrically proportioned quantaties of liquids which react chemically together, for example, a polyol and an isocynate, to a high-pressure mixing head, in which the jets of chemical components are impinged together in a mixing chamber where they are mixed and then fed into a mold; it is also known to provide means for varying and controlling the feeding of one or more components to allow them to be mixed in the correct stochiometric ratios.
Among the various feeding systems currently known, reference is made here to the systems which make use of "absolute" pumping units, that is to say, in the form of cylinders in which a plunger or piston is made to reciprocate in a chamber in order to move, with its stroke, a precise pre-established quantity or volume of liquid; feeding and metering units of this kind are illustrated for example in the IT-A- No. 23119 A/85 of the same applicant, in the DE-A- No. 3309964, in the U.S. Pat. No. 3,912,234 or in the U.S. Pat No. 3,642,175. In all these known devices, the metering units are hydraulically actuated by a source of fluid under pressure and are controlled by means of complex hydraulic or electric systems in order to ensure that the components are fed and mixed in the necessary stoichiometric quantities. However, the use of hydraulic or electric systems for controlling the quantities of liquid fed, which operate in relation to a constantly-sensed physical quantity such as the forward velocity of the plungers of the metering unit, the flow rates and so on, gives rise to margins of error in the metering which depend upon the control system used and which, in any case, call for subsequent operations and adjustments in order to obtain and keep the flow rates in the desired stoichiometric ratios.
From U.S. Pat. No. 2,890,836 and U.S. Pat. No. 3,642,175, there are also known mixing devices, which make use of volumetric pumps or positive pumping devices, interconnected mechanically together; the U.S. Pat. No. 3,642,175 refers to a device for making protective coverings in which the control of the stoichiometric reaction ratios between the components is less critical than in the case of molding mechanical articles having characteristics which must be strictly controlled; consequently, the use of volumetric pumps, with nonabsolute relative efficiencies, even though they are mechanically correlated, does not solve the problem of strict control of the stoichiometric mixing ratios due to the fact that the efficiencies of the pumps differ from one another and may vary with use.
In the U.S. Pat. No. 3,642,175, two feeding cylinders are operated in opposition to each other by means of a rocking rod to feed one component, at a time, at low pressure, into a mixing chamber containing an impeller. If the proposed solution were to be used for feeding several components simultaneously into a high-pressure mixing head, it would require an extremely complicated structure and the use of particular expedients to withstand the considerable stress generated by the chemical components which are usually fed at pressures equal to or over 200 bars.
The result would be an expensive and extremely cumbersome device. Furthermore, a problem common to all the known devices, and especially to those which make use of hydraulically-operated piston pumps, resides in the control of the stoichiometric ratios for the mixing of the various components, which must be carried out with means that are extremely easy to set, which do not require any further adjustment, and which are smooth-running and reliable.
A further problem arising in the known devices concerns the use of means for controlling the plungertype feeding metering units, with which it is possible to achieve a constantly correlated displacement of the plungers of all the feeding units, which are not negatively affected by the parameters of the system, and with which it is also possible to vary the stoichiometric ratios for feeding one or more components, with extremely simple and highly reliable means, so as not to give rise to additional causes of error.
Consequently, the scope of this invention is to provide a device for metering and feeding liquids, as referred to, in order to solve the above-mentioned problems, which is both extremely simple in structure and highly reliable, and which ensures a constant instantaneous control of the quantities fed and of the stoichiometric ratios between the chemical components to be mixed.
A further scope of this invention is to provide a device of the aforementioned type, which can be actuated by a drive motor which has an extremely low energy consumption and installed power, while at the same time ensuring excellent performances and the pressures necessary for feeding the various components to be mixed.
A still further scope of this invention is to provide a device for metering and feeding reactive chemical components to be mixed in a high-pressure mixing head, which is structurally simple and inexpensive, providing performances equal to those obtained with the previously used devices.
By using mechanical control means, according to this invention, which are mechanically interconnected directly to each other, to control metering units of the piston or plunger type capable, that is, of providing an absolute dosage of the chemical components, in which the volume of liquid displaced by the plunger during its forward stroke corresponds at all times to the exact quantity of liquid delivered, it is possible to achieve a precise and constant control of the ratios between the components and to obtain a mixture of components in the correct stoichiometric ratio, which remains constant and independent of any operating parameter of the device.